In recent years, the preparation of crystalline molecular sieves containing framework titanium atoms has received considerable attention due to the fact that such materials are active catalysts for the oxidation of organic substrates. For example, the isomorphous substitution of Si by Ti in a zeolite beta framework by direct hydrothermal synthesis has been reported [Camblor et al., J. Chem. Soc., Chem. Commun. 8, 589-590 (1992); Camblor et al., Zeolites, 13, 82-87 (1993); Corma et al., J. Catalysis, 145, 151-158 (1994); ES 2,037,596 (Camblor et al.)]. The higher activity of such materials as catalysts for the oxidation of bulkier molecules as compared to TS-1 (which is isomorphous with ZSM-5) is attributed to the larger pore size of the zeolite beta structure. Unfortunately, the direct hydrothermal synthesis of aluminosilicotitanates having a zeolite beta structure suffers from a number of drawbacks, including very low yields and the need to use a relatively large quantity of template and water. The development of improved methods for obtaining such catalysts would be highly desirable.
The preparation of molecular sieves containing titanium, silicon, and aluminum atoms in their framework structures is recognized as a highly uncertain and unpredictable art. For example, while European Pat. Publication No. 77,522 claimed the preparation of titano-aluminosilicates having a pentasil (ZSM-5) structure, later workers (Skeels et al.: U.S. Pat. No. 5,098,687) demonstrated that the titanium atoms in the materials obtained were not actually present in the form of a framework tetrahedral oxide. Similarly, although U.S. Pat. No. 4,410,501 contains an example wherein aluminum was allegedly added to a titanium silicate, Skeels et al. subsequently showed that what was actually produced was ZSM-5, not a titano-aluminosilicate having titanium incorporated into the aluminosilicate lattice. As observed in a recent publication [Tuel et al., J. Chem. Soc., Chem. Commun. 1667-1668 (1994)], "the synthesis of Ti-containing materials is relatively difficult."